A fluidized catalytic cracking (FCC) process converts high-molecular weight hydrocarbon fractions of petroleum crude oils to usable products (e.g., gasoline) with the aid of a catalyst in a reactor. High temperature flue gas (e.g., 650° C. to 790° C.) may be produced from the FCC process and passed through a FCC expander to extract and convert energy from the flue gas into mechanical work that may be used to drive process machinery. Although the flue gas is typically processed through multiple stages of separation, an undesirable amount of catalyst particulate typically remains entrained in the flue gas and is passed through the FCC expander.
Accordingly, the high temperatures, corrosive nature, and erosive tendency of the hot, catalyst particulate laden flue gas may cause rapid deterioration by erosion or corrosion of the rotating and stationary components of the FCC expander, including but not limited to, the inlet, the rotor assembly including the rotor disc and blades, and the stator assembly. In particular, the rim of the rotor disc and the respective roots of the rotor blades attached to the rotor disc are susceptible to catalyst build up during operation. This region of the FCC expander is also subject to turbulent and complex flows. Cooling steam may be introduced into the fore, or upstream, side of the cavity in which the rotor disc is disposed, and sealing steam or air may be discharged into the cavity on the aft, or downstream, side of the cavity. Hot, corrosive, and catalyst particulate laden flue gas may be drawn into the cavity, thereby mixing with the cooling steam and contacting the rotor disc proximal the roots of the rotor blades. Additionally, the catalyst may separate from the flue gas and may be deposited on the roots of the rotor blade and on and under the rim of the rotor disc. Corrosion may thus occur, causing the penetration of corrosion spikes into the base material of the rotor blades and rotor disc. Further, mixing of the hot flue gas with the cooling steam increases the temperature of the cooling steam and makes it less effective at cooling the rotor blades and disc, thereby resulting in increased metal temperatures. Such deposits, erosion, corrosion, and/or increased metal temperatures may lead to the reduced reliability and, in some cases, failure of the FCC expander.
Proposed solutions to the deposition, erosion, corrosion, and/or increased metal temperatures have included additional process equipment for the removal of the catalyst particulate entrained in the flue gas, selection of more corrosion resistant materials for the construction of FCC expander components, the use of coatings to improve the corrosion resistance of the base materials of the FCC expander, and the use of additional cooling steam. Each of these proposed solutions have been shown to be effective; however, certain drawbacks are present in each of these proposed solutions. For example, economic considerations may not allow for the additional cost of certain corrosion resistant materials or coating. In addition, the operation of the FCC expander may limit the thickness of the coating, thereby limiting the selection of coating depending on the base material employed. Also, the additional process equipment utilized to further separate the catalyst particulate from the flue gas may enlarge the footprint of the plant or facility, which may be limited in certain environments. Further, the use of additional cooling steam may increase the cost of utility steam consumption and the likelihood of erosion of the rotor disc and rotor blades.
What is needed, therefore, is an improved system and method for cooling the FCC expander components while reducing the erosion and corrosion of the FCC expander components subjected to hot, catalyst particulate laden flue gas produced from the FCC process.